NDS Load Combinations and Duration Factors

[butch81385]

While reviewing problems for the PE, I came across a question regarding wood design. The problem dealt with designing a wood member under Dead Load, Live Load, and Wind Loading. In the solution, they merely added these 3 loads together and used the duration factor. From my research, it seems that if the question is asking to solve only using the NDS, you would just add these loads together (such as with the load combinations in Appendix B of the NDS). However, if you are to use IBC or ASCE 7 in solving the question, you would use those appropriate load combinations (such as DL + 0.75LL + 0.75W).

Does this seem correct? Is there any other reason why one would only add the 3 loads together without using the ASCE 7 load combinations?

Thanks in advance,

Bill

 

[DaveAtkins]

You CAN use the appropriate load combination AND use the 1.6 load duration factor. ASCE 7 is quite clear on this.

DaveAtkins

 

[Splitrings]

I agree. The problem sounds like it is solved incorrectly. NDS 1.4.4 states "Combinations of design loads and forces, and load combination factors, shall be in accordance the the building code under which the structure is designed..."

Although, now that I look at an ASD example in the '05 NDS they add all the loads and divide by the Cd factor for each duration case, to determine which governs. Then they solve the problem with load combintion factors of 1 for dead, live & snow. Not sure..maybe because a specific design code is not referenced they add all the loads.

 

[Splitrings]

I just noticed the 2003 IBC load combinations differ from ASCE 7...I think. If you use Equation 16-9, it is D+L+S. Below it states it is permitted to use a reduction factor of 0.75.